Method for producing magnetic powder



Patented METHOD FOR PRODUCING MAGNETIC POWDER Hans Volt.Berlin-'Steglits, Germany, aslinior to Micro Products Corporation,Hastings on Hudson, N. Y., a corporation of Delaware No Drawing.Application January 27, 1938, Serial No. 61,111. In Germany January 80,1935 9 Claim.

This invention relates to the production of mag-' netic powder. moreparticularly for making high frequency iron' cores. It is an object ofthe invention to render possible the production of such powder fromcheap and easily available initial material. Another object is toimprove the electric properties oi such powders.

Bo far, the only iron powder material giving iron powder cores 01'maximum eiiiciency and minimum losses more particularly for highirequency work and iron cored tuning coils of very small size and highQ-value, was the carbonyl iron. This material, however, is dii'ilcuit toproduce and very expensive. thus handicapping the 16 introduction inradio sets of highly efiicient iron cored tuning coils which aretechnically superior to air coils and to iron cored coils using inferioriron powder. Now, by the novel process, it is possible to make from verycheap initial material I such as red iron oxide etc. and by simple andcheap methods, iron powders which are electrically equivalent if notsuperior to carbonyl iron I and easier to mold to solid cores.

The particular advantage of magnetic powders as made by the presentprocess consists in their high specific resistance due to the siliconcontent; not only is the interior resistance of each particle increasedin this way but also the contact resistance from particle to particle isgreatly increased, so thus reducing eddy currents both within eachparticle and through the whole core structure from particle to particle.Moreover the hardness of the particles isincreased, thus facilitatingthe moulding or compressing of the powder to highly as eiiicient cores.

Further objects of the invention will be apparent from the followingdetailed description. 5 The method according to the invention comprisesintimately mixing small particles contain- 40 ing'iron and aresistance-increasing material such as silicon and/or aluminium andheating this mixture in a reaction chamber under action of non-oxidizinggases to a temperature sufllcient to cause the silicon and/or aluminiumto substan- 45 tially alloy with the iron particles but insufllcient tocause substantial sintering together 01 the iron particles. The siliconcontent in the initial mixture should preferably be more than 3%.

The mixing may be eflected prior to the heat 50 treatment, in mechanicalmills such as ball mills,

centrifugal mills, colloidal mills etc. and gaseous or liquid agents maybe used in the mills so as to protect the particles from oxidizing andto cause intimate mixing and adherence between the dif- 5 ferentparticles. The mixing may also be eilfected or improved by mixing theingredients continuously during the heating and alloying process, forinstance by carrying out the process in a rotary tube furnace or anotherreaction chamber enabling continuous movement and agitation of the 5material.

The initial iron material may be pureiron disintegrated to sufilcientfineness or preferably iron oxide which is reduced in the reactionchamber and simultaneously alloyed with the silicon. Var- 10 ious ironoxides may be used, such as red iron oxide. iron hydroxide (Fe(0I-i)a)having proved to be particularly suitable for this process.

The material containing silicon may be ferrosilicon, such as used iniron foundries and particularly with a silicon content oi the order of98%, which should be ground to very fine particles of the order of 1micron. Also it may be in the form of a silicon compound facilitatingthe alloying process in liquid form, such as silicon tetrachlo- 0 rideand/or in gaseous form such as silicon-hydrogen compounds (silanes). Themixture should be so effected and treated that 4 to 12% of silicon arediflused into the iron particles.

According to the kind of ingredient the nonoxidizing gases used in theprocess may be inert gases, such as nitrogemand/or reducing gases, suchas hydrogen, and/or gases having a catalytic eflect, such as chlorinegas and silanes. For example, if the material already is reduced aneutral gas may be used, but if reduction of the material is necessary areducing gas or a mixture of reducing gas with the neutral gas may beused depending upon the intensity of reducing action required. Thesegases may continuously stream through the reaction chamber, for instancethrough the rotary tube furnace. Further the reaction chamber may bekept under reduced pressure or under elevated pressure. Catalytic gases,and/ or solid or liquid catalytic substances may be added. Silicontetrachloride or other salts, dis,- sociating in the reaction chamberand liberating the silicon, may also be used. Gaseous catalysts may beadded to the hydrogen used in the process.

The electric properties of the powder thus obtained may be considerablyimproved by suitable heat treatment thereof, more particularly by veryslowly cooling down the powder after the process. This may be eflectedby passing the powder from the furnace into a heated vessel, which isvery slowly cooled down and preferably slightly moved.

The initial material should be of such kind and the process so conductedthat the resulting powv der is of great fineness below 20 microns. For

high frequency cores the particles should substantially have a size of 3to 5 microns or, particularly for short waves, even 0.5 to 2 microns.

The powder may be further improved by mechanical treatment after thealloying, e. g. by hammering in such manner that the density of theparticles is augmented and the size reduced. More particularly any lumpsor aggregations are disintegrated. Ball mills using hard balls of highspecific weight may be employed therefore and grinding agents, moreparticularly hydrocarbons, esters, chlorides, solvents etc. may beadded.

Small quantities of insulating substances preferably in dissolved formmay be added which are suitable to form an insulating coating on thesurface of the particles during grinding. This insulating coating may behardened or oxidized or, if it is not hardened, a binder should be usedthe solvent of which does not dissolve the insulating skin.

It is understood that, in the whole process, aluminium or thecorresponding aluminium compounds may be used alternatively oradditionally to silicon and its compounds. Other substances, such ascarbon, boron, titanium, zirconium, thorium, vanadium, antimony,arsenic, phosphorus, tellurium, may likewise be applied alone or incombination with silicon and/or aluminium.

Some forms of the novel process will be described hereunder in detail asfollows:

Example 1 I' e(OH)a is intimately mixed with 2 to 6% by weight ofpowdered silicon and treatedin a rotary tube furnace during about 30minutes at 600 to 800 0., preferably 700, in a preheated hydrogencurrent, to which 0.5% chlorine gas may be added. The iron oxide is thusreduced to iron and immediately alloyed with the silicon. The powder issubsequently cooled .down during 12 hours in a hydrogen atmosphere andtreated in a ball mill-to increase the permeability.

Example 2 Finely disintegrated iron oxide is distributed in a reductionfurnace in a thin layer and heated to GOO-700 C. in a preheated hydrogengas current as under 1), the powder is thus treated during 1 hour andcooled down as under 1). The reduced power is then'ground for 1 to 3hours with about e6% by weight of silicon and heated to 500-700 C.,preferably evacuating the heating chamber. A small quantity of chlorinegas is then added and the powder moved during 30 minutes, then kept atthis temperature during 2 hours and cooled down.

Example 3 100 parts by weight of iron oxide hydrate are intimately mixedwith 2.5% by weight of finely divided 99% ferrosilicon and exposed to ahydrogen gas current containing 1% chlorine gas. during 30 minutes andat 600 C. The powder is cooled down to room temperature during 6 hours;according to analysis, it consists of 4.2% 8i; 95.8% Fe. without freeBi. The same process,

but without chlorine shows 1.7% free Si in the final product. 1

Example 4 100 parts by weight of dry iron oxide hydrate are heated to650 C. in a rotary drum and treated with preheated hydrogen gas to whichbefore entering the furnace 17% by weight of gaseous silicontetrachloride are added during a reaction time of 30 minutes. Furthertreatment as before.

In most practical cases the completeness of reaction can be easilydetermined by compressing the treated powder in a mould without abinder. If the powder forms a coherent body, it is not suitable. while,if falling to pieces, the reaction is finished.

The powder may be moulded to high frequency cores with an insulatingbinder. Mostly, it is not necessary to form an individual insulatingskin upon the particles.

I claim:-

1. A method of producing magnetic powder which comprises heating anintimate mixture of a finely divided solid iron-containing material ofthe group consisting of iron, iron oxides, andiron hydroxides with afinely divided solid material of the group consisting of silicon andaluminum and their alloys in a non-oxidizing atmosphere to a temperaturesumcient to cause the silicon or aluminum to alloy with the iron butinsufiicient to cause sintering together of the particles.

2. Method as defined in claim 1 in which the finely divided iron issupplied to the reaction mixture as such.

3. Method as defined in claim 1 in which the finely divided iron issupplied to the reaction mixture by incorporating an oxidic ironcompound in the reaction mixture and heating the reaction mixture in thepresence of a reducins gas.

4. Method as defined in claim 1 in which the reaction mixture iscontinuously mixed during the heating and alloying operation.

5. Method as defined in claim 1 in which the reaction is carried out inthe presence of a catalytic agent of the group consisting of thehalogens.

6. Method as defined in claim 1 in which the alloy is subjected to heattreatment involving a slow cooling thereof.

'7. Method as defined in claim 1 in which the alloy is subjected tohammering to increase the density and reduce the size of the particlesthereof.

8. Method as defined in claim 1 in which silicon is supplied to themixture as ferrosilicon,

9. Method of producing magnetic powder as defined in claim 1 in whichsilicon is supplied to the mixture as such and iron is supplied to themixture as an oxidic compound of iron and the mixture is heated to atemperature of 600' to 800' C. in a current of hydrogen containing asmall proportion of chlorine and the mixture is slowly cooled andsubjected to impact disintegration.

HANS V001.

